One of the last manufacturing steps in the construction of an organic light emitting diode (“OLED”) is encapsulation. Encapsulation is a way to protect the OLED device from the damaging environmental effects—primarily from oxygen and moisture. It is well known in the art to encapsulate an OLED device by physically mating a top glass (or other suitable material) layer over, but usually not touching, the OLED device with an epoxy border. The glass, together with its epoxy border, typically is an effective, tried-and-true way to provide the necessary environmental protection required for long-lived OLED usage.
Of late, there has been some discussion in the art to provide a cheaper and better manner of encapsulation, called “direct thin-film” encapsulation. In this manner, thin film encapsulation is typically described as a “polymer multi-layer” (PML) comprising alternating and repeating layers of an organic (usually acrylate or the like) and a barrier layer. FIG. 1 depicts a typical PML structure 100 as is currently known in the art. A glass (or other suitable material) substrate 102 provides the support structure for OLED structure 104 formed on top of substrate 102 in any manner known in the art. Layers 102 and 104 form typically the structure that requires encapsulation—either by known techniques or by the encapsulation techniques of the present invention.
Typically, for PML structures, a planarization layer 106 is formed on top of OLED structure 104. Planarization layer 106 typically is an organic layer (e.g. acrylate or the like) and is provided to give a planar surface for the deposition of the PML structure 112a. PML structure 112a typically comprises barrier layer 108 and another planarization layer 110.
Barrier layer 108 typically comprises a sputtered metal, metal-oxide or a dielectric layer. Barrier layer 108 provides the necessary environmental isolation from the corrosive effects of oxygen and moisture. Planarization layer 110 may again comprise an organic layer (e.g. acrylate or the like) and is typically laid down to provide a planar surface for deposition of the barrier layer 108. This entire PML structure 112a may be repeated (e.g. PML structure 112b)—possibly several times—for additional encapsulation of the entire OLED device.
The advantages of direct thin-film encapsulation over the prior art are primarily cost reduction and improved reliability. Using direct thin-film encapsulation, the package may also be thinner and/or lighter and/or mechanically more flexible. Several structures and steps of the prior art may be excluded with this process. For example, there is no need for a separate glass plate, no need for an epoxy seal, no need for a getter (which is typical in the prior art).
One of the problems of the direct thin film encapsulation occurs with the barrier layer. The barrier layer should ideally not contain any point defects (i.e. pin holes) in its surface—otherwise its usefulness as a barrier layer is severely compromised. That is primarily the reason that a planar organic layer is typically used as a substrate upon which the barrier layer is deposited.
This problem is exacerbated during the batch fabrication of many OLED devices upon a single large sheet of glass—such as shown as a top view in FIG. 2. Upon such a single glass sheet 200, several tens (or even hundreds) of OLED devices 202 may be so fabricated. As depicted, OLED devices 202 are typically laid down in rows and columns on a large sheet of glass 200. Typically, each OLED 202 comprises an electrical contact area 204 for electrically mating the OLED device to a driver circuit.
At the thin film encapsulation step, the PML structure is deposited where at least one UV-curable organic liquid material is deposited over the entire glass sheet containing the multiple OLED devices. This organic layer is subsequently cured—followed by a deposition of a barrier layer (e.g. sputtered metal-oxide or dielectric). Such a process may be repeated to form a PML structure—primarily to avoid external particle/dirt-induced pinhole defects. After encapsulation, singulation is performed, for example by forming scribe and break lines 206 upon the entire structure so that individual OLED devices 202 may be separated and further processed.
The problem with this PML technique is that the only part of the device that requires encapsulation is the OLED structure itself—and not, e.g., the electrical contact pads. In fact, the contact pad must typically be exposed for electrical mating with external driver circuitry. So, at a minimum, additional processing must be performed for the removal of the PML structure over these areas.
Another potential problem with the current PML techniques is that by having the PML layer over the scribe and break lines and/or the glue lines the integrity of the sealed package may be deteriorated, for example by delamination of the PML layer over these areas.
Approaches other than PML are known in the prior art that use combination of organic planarizing layers together with inorganic barrier layers to achieve some degree of thin-film direct encapsulation. Organic planarization layers that do not require special cure may be used as well as layers that are electron-beam or thermally cured, in vacuum or gas atmosphere, preferably inert gas. Such organic layers may also be deposited in non-liquid form, e.g. be evaporated or plasma-deposited (e.g. Parylene).
Monomers can be used as the organic planarizing layer. The use of monomers in contact with an active area of an OLED (the active area may be, for example, the area defined by the cathode) can result in the contamination of the OLED (e.g., the OLED develops pin holes). The contamination can occur because the monomer can diffuse before it is cured and migrate through the pinholes and around the edges of the active area. The monomer may not completely cure so there remains a small proportion of uncured monomers that slowly attack the OLED. To overcome this problem, the prior art uses monomers that immediately react upon contact with a surface such as the active area of the OLED or the substrate. The immediate reaction on contact with, for example, the active area results in the monomer not being able to contaminate the OLED through defects (e.g., pin holes) in the active area. The problem with using monomers that immediately react is that since they disperse everywhere, there is no opportunity to pattern the organic planarization layer. Therefore, it is desirable to have a planarization layer that can be patterned and that minimally contaminates the OLED.
If the method used to deposit the barrier layer onto a device is reactive, then the deposition of the barrier layer may damage the organic electronic device that is to be encapsulated. To avoid such damaging reactions, the planarization layer is deposited using less reactive methods such as evaporation, screen printing, or ink-jet printing. However, if solvents are used to form uniform films of the planarization layer, then these solvents may react with the device to be encapsulated resulting in damage to that device. Also, the planarization layer itself may react with the device to be encapsulated although not as much as the barrier layer deposited by a reactive method. Therefore, it is desirable to deposit a planarization layer that minimally reacts with the device to be encapsulated while still performing the functions of the planarization layer such as minimizing the effects of dirt particles and pinholes.